1. Field of the Invention
The present invention relates to a negative electrode active material which can occlude and release lithium and which has a high reversible capacity and lowered initial irreversible capacity, and its manufacturing method. The present invention also relates to a lithium ion secondary battery using the negative electrode active material.
2. Description of the Related Art
As a power source of information devices such as a cell phone and a notebook-sized personal computer, a lithium ion secondary battery using a nonaqueous electrolytic solution with high energy density is widely used, but to deal with increased power consumption due to the higher performance of these information devices and the increased volume of content which these information devices handle, a higher discharge capacity of a lithium ion secondary battery is sought after. Moreover, in terms of the need to decrease oil consumption, alleviate atmospheric pollution and the emission abatement of carbon dioxide which causes global warming, expectations are growing in regard to low-emission vehicles such as electric vehicles and hybrid vehicles which can act as substitutes for gasoline-engine vehicles and diesel-engine vehicles, and as a battery for driving an engine in these low-emission vehicles, a large lithium ion secondary battery which has high energy density and output density, and therefore has high capacity density, is desired.
The currently prevailing lithium ion secondary battery using a nonaqueous electrolytic solution uses a lithium layered compound such as lithium cobalt oxide (LiCoO2) as a positive electrode active material, graphite, which occludes and releases lithium as a negative electrode active material, and a solution in which a lithium salt such as lithium hexafluorophosphate (LiPF6) is dissolved in a nonaqueous solvent including ethylene carbonate and propylene carbonate. To attain a higher capacity in these lithium ion secondary batteries, it is necessary to increase the quantity of lithium which is occluded in and released from a negative electrode active material. However, since the theoretical capacity of graphite calculated from LiC6 in which the maximum amount of lithium is occluded is 372 mAhg−1 and the existing secondary battery already has a quantity close to the theoretical capacity, the usage of a negative electrode active material as a substitute for graphite is indispensable to attain higher capacity in a lithium ion secondary battery. Moreover, the substitute must exhibit stable characteristics under a charge-discharge cycle.
As a graphite substitute with high capacity, a metal which forms an alloy with lithium, such as aluminum, zinc and tin are exemplary. Especially, Sn is suitable because its theoretical capacity calculated from Li4.4Sn is as high as 994 mAhg−1. However, it has a problem in that volume expansion of Sn in accordance with occlusion of lithium is remarkably large. If the volume of Sn is 100%, the volume of Li4.4Sn is as much as 358%. Therefore, if the charge-discharge cycle is repeated in a battery in which Sn is used as a negative electrode active material, a crack is produced on the negative electrode due to too big a volume change in accordance with the occlusion and release of lithium, an electron conducting path which is indispensable for a charging and discharging reaction, is destroyed, and only several repetitions of charge-discharge cycle will sharply decrease the discharge capacity.
To solve this problem, a method to alleviate the stress due to the volume change of Sn by dispersing Sn in a matrix of carbon materials or oxides has been suggested. As a method to disperse Sn in the matrix of an oxide, there is a method to use tin dioxide as an active material (see Non-Patent Document 1 (Journal of Power Sources 159 (2006) 345-348 and Non-Patent Document 2 (CARBON 46 (2008) 35-40).
Tin dioxide occludes lithium by reaction as set out in the following chemical equations (I) and (II). The reaction in equation (I) in which reducing of tin dioxide and formation of lithium oxide occurs is referred to as a “conversion reaction” and the reaction in equation (II) in which an alloy of Sn and lithium is produced is referred to as an “alloying reaction”. It is considered that lithium oxide produced from the conversion reaction acts as a matrix of Sn, which alleviates the stress due to the volume change of Sn in the alloying reaction region and inhibits aggregation of Sn in the alloying reaction region.SnO2+4Li++4e−→2Li2O+Sn  (I)Sn+4.4Li++4.4e−Li4.4Sn  (II)
However, the conversion reaction set out in equation (I) was conventionally said to be an irreversible reaction because lithium oxide was thermodynamically stable. Therefore, in a lithium ion secondary battery in which tin dioxide was used as a negative electrode active material, only the alloying reaction region (within the range of 0 V to approximately 1 V against a Li/Li+ electrode), which is a reversible reaction region, was used, and if charging/discharging was carried out by widening the potential range to an electric potential including the conversion reaction region, a great initial irreversible capacity due to the irreversibility of the conversion reaction was observed.
For this problem, the applicants suggested, in WO2011/040022 which was published after the filing date of an application regarded as a basis of the claim of priority in this application, a negative electrode active material in which the conversion reaction that was considered to be an irreversible reaction reversibly proceeds. The negative electrode active material shown in WO2011/040022 is a negative electrode active material, in which tin oxide powder and nanosize conductive carbon powder are comprised in a highly dispersed state. If the nanosize conductive carbon powder is used, the conversion reaction, which used to be regarded as an irreversible reaction and the cause of large initial irreversible capacity, proceeds reversibly, and therefore the conversion reaction region as well as the alloying reaction region can be used for the occlusion and releasing of lithium.
The reason why conversion reaction has now been observed to progress reversibly is not obvious at this moment, but it is considered to do for the following reasons. In the nanosize conductive carbon powder, oxygen atoms (oxygen in a surface functional group such as a carbonyl group and a hydroxyl group, or adsorbed oxygen) are contained abundantly, and therefore, a Sn—O—C bond through the intervention of this abundant oxygen becomes prone to be produced. Moreover, lithium oxide produced in the conversion reaction is considered to exist in a metastable state as illustrated in the following formula (III). It is considered that, as a condition in which lithium is easily removable from this lithium oxide in the metastable state is produced, formation of tin oxide simultaneously with removal of lithium is likely to occur, and the conversion reaction generates reversibly. Besides, if the nanosize conductive carbon powder and the tin oxide powder exist in a highly dispersed state, Sn—O—C bonds are formed in many sites because the points of contact of carbon powder and tin oxide powder increase, and therefore, the metastable state of the formula (III) is formed in many sites after the conversion reaction. As a result, a charge-discharge cycle within the range from 0 V to approximately 2 V against a Li/Li+ electrode can be realized, and discharge capacity can be significantly increased.

With this negative electrode active material comprising tin oxide powder and nanosize conductive carbon powder in a highly dispersed state, the conversion reaction region as well as the alloying reaction region can be used for occlusion and releasing of lithium, but as a result of further investigation, there is an initial irreversible capacity which seems to result from electrochemical decomposition of an electrolytic solution on the surface of carbon powder. This initial irreversible capacity is not preferable because it requires more positive electrode active material to be incorporated in a lithium ion secondary battery produced by combining this negative electrode active material with the positive electrode active material and, in a cell with a given volume, the quantity of the negative electrode active material is reduced all the more and the capacity per cell becomes small.
The applicants undertook repeated investigations, and in PCT/JP2012/054458, which was not published at the time of this application, suggested a negative electrode active material in which nanosize conductive carbon powder and tin oxide powder in contact with the surface of the conductive carbon powder are comprised in a highly dispersed state, and also a metal oxide other than tin oxide that contacts the surface of the conductive carbon powder, and/or, a low-conductive amorphous carbon layer that covers the surface of the conductive carbon powder were further comprised.
In the negative electrode active material disclosed in PCT/JP2012/054458, reversible progression of the conversion reaction is maintained. Moreover, probably because the active site on the surface of the conductive carbon powder that catalyzes the electrochemical decomposition of an electrolytic solution is coated by the metal oxide other than tin oxide and/or the low-conductive amorphous carbon layer and the electrochemical decomposition of an electrolytic solution is inhibited, initial irreversible capacity is reduced. The surface condition of the amorphous carbon layer that covers the surface of the conductive carbon powder is the same as the surface condition of the conductive carbon powder, but since the amorphous carbon layer has low conductivity, the electron necessary for the electrochemical decomposition of an electrolytic solution becomes difficult to be supplied to the surface of the amorphous carbon layer, and therefore, the electrochemical decomposition of an electrolytic solution on the surface of the amorphous carbon layer is inhibited. As a result, reduction of the initial irreversible capacity while maintaining the high reversible capacity of the negative electrode active material is possible.